Both thallium reinjection and 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) protocols have been shown to correlate and predict functional recovery of asynergic regions after revascularization. While such good correlation between thallium reinjection and FDG PET is encouraging, there is no quantitative, histomorphologic "gold standard" in human subjects that validate the viability information of thallium and FDG. Therefore, we prospectively studied 15 nondiabetic, chronic ischemic cardiomyopathy patients listed for cardiac transplantation, with stress-redistribution thallium tomography and FDG PET at rest. Eight pts have received cardiac transplantation, of whom 6 have had histomorphologic analysis by 2 experienced pathologist blinded to the thallium and PET results. The explanted hearts were sliced in short-axis sections (mean thickness 7 mm) and the volume fraction of myocytes from midventricular slices were studied quantitatively after staining with Picro Sirius red. Each tomogram was divided into 8 regions and the percent area of viable myocytes in each region was calculated separately. For thallium redistribution, reinjection, and FDG PET studies, the myocardial region with maximum counts on the thallium stress image series was used as the normal reference region for that patient. Among the 48 matched histomorphologic, thallium and FDG regions analyzed, while there was a good correlation between % thallium uptake on redistribution (mean 71.5 plus/minus 22.3) and % viable myocytes by histomorphologic analysis (mean 74.9 plus/minus 15.4, r=.56, pless than0.001), the correlation was improved with thallium reinjection (mean 73.7 + 23.7, r=.67, plessthan0.001) and FDG PET (mean 76.3 plus/minus 19.4, r=.67, p less than0.001). These data provide histomorphologic confirmation that the magnitude of thallium and FDG uptake correctly estimate the distribution of viable myocytes in human subjects.